This invention relates generally to a resin composition hardenable by irradiation of a high energy ray and, more specifically to a resin composition suitable for plugging through-holes of a metal-plated board in manufacturing printed wiring boards.
Recent development of electronics art demands printed wiring boards having a high circuit density. Thus, printed wiring boards provided with wiring patterns on both sides thereof are now increasingly used. One well known method for the production of such both-sides wiring boards includes the steps of: drilling or punching a board to form a multiplicity of through-holes, plating copper on both sides of the board and on inside walls of the through-holes, plugging the through-holes with an alkali-soluble resin, curing or hardening the resin within the through-holes, abrading or sanding both surfaces of the metal-plated board to remove a layer or deposit of the resin thereon, screen printing positive patterns with an alkalisoluble resist, curing the resist patterns, etching the exposed metal layer with an acidic etching liquid, and removing the resist patterns and the plugged resin using a 2-3% aqueous sodium hydroxide solution.
The plugging of the through-holes is generally performed as follows. A curable or hardenable resin is first applied to the board by means of, for example, a roll coater to plug the through-holes. An excess resin on the surface of the board is then removed by a squeegee and the resin plugged in the through-holes is heat-treated or UV-treated for effecting curing or hardening. The resulting board is then sanded or abraded by means of a buff roll or a belt sander to remove a layer of the hardened resin, which has been failed to be removed during the treatment with the squeegee, and to clean the surface of the metal-plated board.
As a resin for plugging through-holes of the board, there has been used a heat-curable or a photo-curable resin. Since a photo-curable resin can be hardened within a shorter period of time as compared with a heat-curable resin and since the former resin is substantially free of change of volume upon hardening, recent trend is toward the use of the former resin. The former resin, however, poses problems because only a surface portion of the resin within the through-holes is hardened when irradiated by UV rays and because the hardened resin is not easily dissolved in an alkali solution.
More particularly, with the known UV-hardenable resin, the hardening occurs only in a depth of about 1/3 from both surfaces of the through-hole and the remainder of the intermediate 1/3 portion remains still unhardened. As a result, the resin which has not yet been hardened is often exposed on the surface during the abrading steps to cause various problems in the succeeding steps. In the step of removing the hardened resin by spraying an aqueous alkaline solution, such as a 3% aqueous sodium hydroxide solution, which is to be carried out after the formation of wiring pattern by etching, about 10 minutes are required for completely dissolving the hardened resin in the solution even when this treatment is performed at about 45.degree. C.
With a view toward overcoming the above problems, there has been proposed in Japanese Published Unexamined Patent Application No. 62-59606 an improved resin composition containing the following ingredients:
(a) a resin selected from the group consisting of rosins and modified rosins and having an acid value of at least 150 and a Gardner color scale of not greater than 15; PA1 (b) a compound of the general formula (I): EQU Y--CO--O--A--O).sub.m CO--CR.dbd.CH.sub.2 (I) PA1 wherein Y represents ##STR2## A represents an alkylene having 1-4 carbon atoms, R represents hydrogen or methyl and m is an integer of 1-4; PA1 (c) a hydroxyl group-containing compound having only one acrylic or methacrylic group and at least one hydroxyl group. PA1 (a) a resin selected from the group consisting of rosins and modified rosins and having an acid value of at least 150 and a Gardner color scale of not greater than 15; PA1 (b) a compound of the general formula (I): EQU Y--CO--O--A--O).sub.m CO--CR.dbd.CH.sub.2 (I) PA1 wherein Y represents ##STR3## A represents an alkylene having a 1-4 carbon atoms, R represents hydrogen or methyl and m is an integer of 1-4; PA1 (c) a hydroxyl group-containing compound having only one acrylic or methacrylic group and at least one hydroxyl group; and PA1 (d) a polyol compound having at least two hydroxyl groups. PA1 The term "acid number" used in the present specification in connection with rosins or modified rosins is defined as the amount (mg) of potassium hydroxide required for neutralizing 1 g of the sample. The acid number is measured in accordance with JIS K5400 by titration of a solution of the sample in toluene, methyl ethyl ketone or the like solvent using a 0.1 N aqueous potassium hydroxide solution and phenolphthalein as an indicator.
While the hardening of this composition proceeds satisfactorily not only in surface portions but also in a middle portion of the through-holes and the period of time required for removing the hardened resin by contact with an alkaline liquid is about a half of that required in the case of using the above conventional resin composition, this composition still encounters a problem because the impact strength of the hardened resin is not high. Thus, during the abrasion step for removing excess hardened resins on the surface of the board, cracks tend to be formed in the hardened resin bodies within the throughholes, which also causes breakage of the plugging resin bodies. When such cracking or breakage occurs, the plated layer inside of the through-holes are attacked by an etching solution in the succeeding stage, causing electrical disconnection between the wiring patterns on the both sides of the board.